1. Field of the Invention
The present invention relates to apparatuses for analyzing a stack gas produced by combustion, and more particularly to a filter probe for sampling components of a stack gas which employs a non-clogging flow through inertial filter and means for conditioning the stack gas sample to optimize the analyzation thereof.
2. Description of the Contemporary and/or Prior Art
The desire to analyze stack gases produced by combustion processes is well known in the art. Typically, a sample extracted from the combustion process is drawn off by a sampling probe which is installed in a stack. The output of the probe then connects to sample line leading to remotely located analyzers. Depending upon how the probe is designed and how analysis takes place, the gases may need to be filtered, cooled, and dried in sample conditioning stages before being presented to the analyzers. Two of the currently employed major probe design styles differ as to whether the sample is sent down the line in an as is condition, that is hot, wet and at a full concentration or whether the sample is diluted with a neutral carrier gas, usually clean instrument air. Further, extraction probes usually include some kind of filtering medium so as to avoid loading the sample line with dust. One such probe includes a "inertial" bypass filter element which consists of a ceramic or metallic porous wall tube through which the gas extracted from the stack is drawn at a high speed. This element is attached to the outlet end of the probe which in itself is just a piece of straight pipe and is followed by an air driven eductor or jet pump to provide the suction for moving the gases through the probe and inertial filter, without the danger of clogging the straight inline bore of this device. A small portion of the total gas flow through the filter element permeates through the filter walls and is collected in a surrounding jacket from which it is drawn by a sample line to subsequent stages for analysis.
Particles entrained in the high velocity axial gas flow are precluded from deposition on the porous filter walls or from penetrating therethrough by the ballistic effect of particle inertia, hence the name "inertial" bypass filter. The low radially velocity of such devices also inhibits particles from penetrating the porous wall. Such devices are manufactured by Mott Metallurgical Corporation, Farmington Industrial Park, Farmington, Conn. and are disclosed in their brochure entitled Inertial Gas Sampling Filter Systems DB 4600.
A disadvantage of currently known diluting stack samplers is that an average of the analyzed components is presented rather than an instantaneous reading.
Another type of extraction probe employs a filter in the tip of the probe followed down stream by a dilution mixer made up of a critical flow orifice and eductor. The critical flow orifice limits the stack gas flow and is sized in conjunction with the air driven eductor in such a manner that the flow ratio between the orifice flow and the eductor air set up a defined dilution ratio.
This ratio is usually picked in the range of between 15 to 1 and 300 to 1 depending on the type of analyzer served. As a result, the hot stack gas is immediately diluted and therefore the sample has a reduction in temperature and moisture content which eliminates the need for additional sample conditioning before the gas is fed to the analyzers. An unfortunate disadvantage of this configuration is that the tip filter in the probe frequently clogs leading to frequent probe maintenance and the instability of the dilution ratio that depends on the uncertain relationship between the critical orifice and the eductor flows as a function of variations in the absolute stack pressure. Such a diluting stack sampler for monitoring gaseous emissions is manufactured by Environmental & Process Monitoring of Prospect, Illinois and is described in the Manual for Model 797 Diluting Stack Sampler.